PAOs are recognized as a class of materials that are exceptionally useful as high performance synthetic lubricant basestocks. They demonstrate excellent flow properties at low temperatures, good thermal and oxidative stability, low evaporation losses at high temperatures, high viscosity indexes, good friction behavior, good hydrolytic stability, and good erosion resistance. PAOs are miscible with mineral oils, synthetic hydrocarbon liquids, and esters. Consequently, PAOs are suitable for use in engine oils, compressor oils, hydraulic oils, gear oils, greases, and functional fluids.
Olefin polymerization catalysts are of great use in industry. Hence there is interest in finding new catalysts that increase the commercial usefulness of the catalyst and produce polymers having improved properties. Various catalysts exist for the polymerization of olefins. Friedel-Craft catalysts, such as aluminum trichloride or boron trifluoride, and a protic promoter have traditionally been used. The demand for high quality PAOs has been increasing for several years, however, driving research into alternatives to the Friedel-Craft process.
Metallocene catalyst systems are one such alternative. Most of the metallocene focus has been on high-viscosity-index-PAOs (HVI-PAOs) and higher viscosity oils for industrial and commercial applications. Examples include U.S. Pat. No. 6,706,828, which discloses a process for producing PAOs from meso-forms of certain metallocene catalysts with methylalumoxane (MAO). Others have made various PAOs using various metallocene catalysts not typically known to produce polymers or oligomers with any specific tacticity. Examples include U.S. Pat. Nos. 5,688,887; 6,043,401; 5,087,788; 6,414,090; 6,414,091; 4,704,491; 6,133,209; 6,548,724; and 6,713,438.
A relatively smaller amount of focus has been on making PAOs using a specific type of metallocene catalyst comprising only one cyclopentadienyl (“Cp”) ring. These metallocene catalysts are often referred to as “mono-Cp” or “half-sandwich” metallocene catalyst compounds. Mono-Cp metallocene compounds are compounds in which one of the Cp ligands has been replaced by an anionic ligand fulfilling similar coordination-chemical purposes. The active species in bis-Cp metallocene compounds is the 14-electron cation [Cp2MR]+. The replacement of one Cp ligand by an alkyl group in a mono-Cp compound provides the 10-electron cation [CpMR2]+as the active species. Such a species is more electron deficient, sterically less hindered, and possesses two, rather than one, vacant coordination sites for monomer binding. As such, the catalytic behavior of mono-Cp metallocene compounds is generally expected to be different from related bis-Cp metallocene compounds.
The additional Cp group as a ligand for bis-Cp metallocene catalysts stabilizes the active catalyst site and exerts steric and electronic influences that affect catalyst performance and characteristics of the PAO formed. Some aspects of the role of this second Cp ligand may be filled by other ligands, however, different ligands may cause otherwise similar catalysts to operate differently. Mono-Cp titanium-based metallocene compounds comprising a bridging group, aromatic group, and additional R2 ligands were disclosed as polymerization catalysts in J Saβmannshausen et al., J. Organomet. Chem. 1999, 592, 84-92. However, the reference indicates that the effect of a comparatively weakly coordinated pendant ligand, such as phenyl, on the behavior of the polymerization catalyst is difficult to predict. U.S. Pat. No. 7,056,995 discloses the use of similar catalysts for the trimerization of low molecular weight olefins, primarily ethylene, to selectively obtain 1-hexene, but does not demonstrate that these catalysts can be used to obtain higher molecular weight PAOs. Other examples of Mono-Cp catalysts include U.S. Pat. No. RE40234E (mono-Cp compounds useful for producing crystalline PAOs, particularly polypropylene), U.S. Pat. No. 5,043,515 (unbridged mono-Cp catalysts comprising zirconocene and one or more alkyl aluminoxanes, primarily useful for the polymerization of low molecular weight monomers, specifically ethylene), and U.S. Pat. No. 7,279,609 (mono-Cp catalysts useful for alpha-olefin trimerization comprising extractable counter-ions which are preferably halogens, especially chlorine). Additional examples include U.S. Pat. Nos. 5,171,919 and 6,680,417 and U.S. Patent Application Publications 2004/0038806; 2006/0183631; and 2009/0036621.
There is still a need for new and improved catalysts for the polymerization of olefins to achieve specific olefin properties, increase conversion rates, improve efficiency or economics, and/or provide greater industry flexibility. Specifically, there is a need for new processes and catalysts for making high viscosity PAOs with narrow molecular weight distribution (MWD), this latter attribute indicative that the PAO will demonstrate improved shear stability in lubricant formulations. Shear instability may lead to permanent viscosity losses in lubricants, which can have an undesired impact on lubricant performance and life, equipment downtime, and equipment life. The present invention is directed to new catalysts useful for producing high viscosity PAOs with improved shear stability.